The present invention relates to the measurement of system parameters in a cable television (CATV) system, and more particularly to a method and apparatus for in-service measurement of composite triple beats.
A cable television (CATV) system passes a conglomerate of typically sixty or more television signals through a common distribution system which includes active elements, such as amplifiers. These active elements are imperfect in several ways, including being somewhat non-linear. Third order terms in the nonlinearities are particularly troublesome because they tend to produce numerous, commonly several thousand, spurious signals in the immediate vicinity of each channel's visual carrier. The ensemble of all these spurious signals associated with a particular channel is known as a "composite triple beat" (CTB). The amplitude of the CTB grows very rapidly with signal levels. It is considered necessary to adjust the signal levels so that the CrB's root-mean-square (RMS) amplitude is at least 51 decibels (dB) below the visual carrier's peak envelope amplitude pursuant to Federal Communications Commission (FCC) regulations.
Another major adverse factor in CATV systems is the buildup of noise. As one looks further and further out in the distribution system from the headend, the total noise amplitude increases because of additive contributions from each amplifier along the chain.
Thus in a CATV system there is a tradeoff in setting operating signal levels. Too high a signal level results in an excessive CTB level, which causes crawling, flickering, horizontal bands, etc. in the picture. Too low a signal level results in poor signal-to-noise (S/N) ratio, which causes a snowy picture. The acceptable amplitude range becomes very narrow in large systems, and CATV engineers tend to become preoccupied with measurements of CTB and noise levels in their distribution system. Their concerns arise partly out of the need to determine the best operating signal level and partly from the need to document compliance with statutory limits on signal defects.
Individual components of each CTB are due to the mixing of various combinations of three different signals. Given the channel frequency arrangement and allowable frequency errors, it turns out that most of the CTB energy on a given channel is contained in a band about 30 KHz wide centered within 15 KHz of the visual carrier. There are also potentially significant CTBs in similar bands at 2.5 MHz above and 2.5 MHz below the visual carrier. However these CTBs are generally weaker and are less bothersome to the viewer due to their large offset from the visual carrier, although there may be some correlation between these CTBs and the CTB about the visual carrier.
In principle the measurement of CTB amplitude is straightforward: just shut down the carrier of the channel of concern and measure the CTB amplitude with a spectrum analyzer, using a resolution bandwidth (BW) sufficient to encompass the entire CTB, usually 30 KHz. Then restore the carrier, measure its amplitude and calculate the ratio of the two measured values. Unfortunately there are some major difficulties with this approach:
CATV subscribers are not amused when their signal disappears for any reason. PA1 Current Federal Communications Commission (FCC) regulations require that the CTB level be measured at the output of the set top converter commonly used by subscribers. If the carrier of the channel under test is simply shut down, the automatic gain control (AGC) of the converter drastically increases the converter's gain, so that there is now considerable difficulty in accurately comparing the CTB amplitude to that of the visual carrier that normally exists. PA1 Most converters currently in use work by demodulating the selected channel to baseband video, then eventually remodulating the video back onto a fixed frequency radio frequency (RF) carrier. In the absence of a suitable carrier at its input, the response of the converter to an essentially noise-like signal is such that CTB energy becomes ill defined at best.
In view of these difficulties it becomes very desirable to have a means of measuring the amplitude of the CTB in the presence of the normal television signals.
The National Cable Television Association (NCTA) has published recommended practices for measurements on cable television systems. One of the recommended methods from the October 1993 revision, incorporated herein by reference, for measuring the CTB eliminates the need to remove the carrier and uses a spectrum analyzer as a fixed tuned receiver, but it does require removing the modulation on the carrier. This method is helpful if communications with the headend are not available or the carrier is not easily controllable. The CTB appears as an amplitude modulated (AM) component on a continuous wave (CW) carrier, and by comparing this AM component with the carrier level, the CTB is measured. However on some types of CATV systems the CTB appears as a CW signal and is not measurable with this method. Also hum and cross modulation appear as AM components, and are thus indistinguishable from the CTB using this method,.
A method of in-service cable television measurements is disclosed in U.S. Pat. No. 5,073,822 issued Dec. 17, 1991 to Linley F. Gumm, incorporated herein by reference. This technique uses a modified spectrum analyzer for determining both carrier-to-noise and CTB ratios. The modified spectrum analyzer is triggered for single sweep during the vertical interval of a selected channel and the noise amplitude, peak sync tip amplitude and frequency of the channel are determined. For CTB measurements a special headend unit is coupled between a television signal source and a combiner prior to distribution to offset a selected channel by a predetermined frequency for a specified number of lines in the vertical interval. The peak sync tip amplitude and CTB amplitude are measured by the modified spectrum analyzer during the specified number of lines. The CTB ratio is determined from the CTB amplitude and the peak sync tip amplitude and displayed. This technique requires a special unit at the headend as well as communication with the headend.
What is desired is an in-service measurement of composite triple beat in a cable television system that is accurate and may be performed at a set top converter without the requirement of communication with the headend or of special headend equipment.